International actors blamed each other Sunday during an urgent UN Security Council as a Syrian city of nearly 275,000 residents descends into new levels of warfare.

Aleppo is being pummeled in a joint operation by Russian and Syrian forces that began three days ago, after a seven-day truce ended failed to deliver any humanitarian relief to Syrians.

"A so called military solution or victory is impossible in Syria," the UN’s Syria envoy Staffan de Mistura told the Council in his report.

Holding out hope, he said a "tiny window of opportunity still exists" for the U.S. and Russia to come to an agreement for a cease-fire.

U.S. Ambassador to the UN Samantha Power said at least 139 people have been killed in more than 150 airstrikes in three days of the Aleppo offensive, describing the situation as "obscene" and "apocalyptic".

Power had harsh words for Russia, a permanent member of the Council, accusing it of "barbarism.

"Russia has the power to stop this suffering," she said. "What you can be sure that Russia will not do is accept responsibility for its actions."

Power criticized Moscow for doublespeak, saying the first airstrikes of the Aleppo offensive, which she made a point of saying was a joint Russian-Syrian operation, occurred during the annual UN General Assembly meetings in New York last week, in which Russia expressed a commitment to end the violence in Syria.

Rycroft called the offensive an "unprecedented unrelenting onslaught" that is "reducing Aleppo to rubble.

"It is difficult to deny Russia is partnering with the Syrian regime to carry out war crimes," he said, adding that Russia "has no credibility left on these issues".

Rycroft said Russia and the Syrian regime have killed more people than Daesh and al-Nusrah combined. The UN has designated both groups terror organizations.

Rycroft said bunker buster bombs, used to penetrate targets buried underground, were being used on civilian settlements in Aleppo, in addition to targeting water supplies and utilizing incendiary weapons.

Power said some of the first airstrikes on Aleppo three days ago targeted and decimated White Helmets, a first-responders aid group that rescued Omran Daqneesh, the five-year-old Syrian child whose face covered in dust and blood became an icon of civilian suffering in the war-torn nation.

Russian Ambassador to the UN Vitaly Churkin responded by contending that Moscow has "always sought to compromise" on cease-fire issues, describing a peace effort "an almost impossible task".

Seemingly unfazed by the barrage of criticism from other permanent members of the Security Council, Churkin commended the regime of Syrian President Bashar al-Assad for displaying "enviable restraint" and slammed the U.S. for dragging its feet in reining in moderate opposition groups fighting Assad.

Churkin said the misguided U.S.-led anti-Daesh coalition attack that killed 62 Syrian troops was carried out by a U.K. plane.